It is known that field effect transistors (FETs) may be used as sensors, e.g. by functionalizing the gate electrode electrode of the FET, e.g. by coating with or forming it from a substance that can interact with an analyte of interest, such that the operation of the FET becomes sensitive to chemical compounds or particles of interest. For instance, the current flowing through the FET in such applications may be typically dependent on the concentration of the chemical compound of interest in a medium brought into contact with the gate electrode electrode of the FET.
Several different types of FETs have been developed for this purpose, such as a ChemFET, which is typically sensitive to chemical compounds, and an ISFET, which is sensitive to specific types of ions. One of the benefits of having FET-based sensors is that the sensor may be integrated in an integrated circuit (IC), such that the sensor signal processing components can be provided on the same die, thus yielding a compact arrangement.
Another example of a FET-based sensor is the extended gate electrode FET (EGFET), which is shown in FIG. 1. The FET 30 is formed in a substrate 10, and has a gate electrode 32 that is connected to a measurement electrode 36 via a conductive connection 34 through for instance a protective layer 20 such as a dielectric layer for protecting the circuitry on the substrate 10 from exposure to the medium 50. A reference electrode 40 is also present. Both the measurement electrode 36 and the reference electrode 40 are in contact with a medium 50 containing the analyte of interest in a concentration to be determined from the current through the FET 30 as controlled by the gate electrode voltage, which, due to the connection 34 between the gate electrode 32 and the measurement electrode 36 is a reproduction of the potential induced by the analyte of interest being sensed by the measurement electrode, e.g. through chemical binding with the measurement electrode plus the potential (bias) on the reference electrode 40. The FET 30 will start exhibiting a current flowing though the device when the gate electrode voltage exceeds the threshold voltage VT of the FET 30. The FET 30 is referred to as an EGFET because of the fact that the measurement electrode 36 acts as a gate electrode extension of the FET 30.
One of the problems with a sensor as shown in FIG. 1 is that the potential difference between the measurement electrode 36 and the reference electrode caused by the medium 50 may not be large enough to exceed VT. In such a scenario, the sensor may have insufficient sensitivity. A straightforward solution to this problem is to bias the gate electrode 32 and the measurement electrode 36 by applying a (DC) voltage to the reference electrode 40, thereby effectively lowering VT. This, however, has the drawback that a current may run through the medium 50 if the measurement and reference electrodes in the analyte and surrounding surfaces are not insulated properly, which may cause unwanted electrochemical reactions resulting in sensor drift or corrosion.